Gene expression in prokaryotic and eukaryotic cells is regulated on the transcriptional and translational levels. For trancription to occur, RNA synthesis is catalyzed by the enzyme RNA polymerase. Trancription starts when RNA polymerase binds to a special region, the promoter, at the start of the gene. The promoter usually precedes the first base pair that is transcribed into RNA, the startpoint. From this point, RNA polymerase moves along the template, synthesizing RNA, until it reaches the termination sequence. This action defines a transcription unit on the DNA molecule that extends from the transcription initiation site (startpoint) to the terminator.
Regulation of gene expression on the transcriptional level occurs by various mechanisms. Gene expression is controlled by particular regulatory sequences, such a promoters and enhancers, to which cellular factors may bind and thereby alter the expression rate of the adjacent gene. Such cellular factors include, for example, so-called transcription factors, which are proteins required for the recognition by RNA polymerases of specific binding sequences in genes.
Certain applications of recombinant DNA technology require that a gene be tightly regulated by its promoter, ie. that the transcription level of the gene is not dependent on any cis-acting elements other than the promoter itself. For example, in the context of gene therapy, the tissue selectivity of a viral vector administered for a therapeutical purpose may rely on the specific regulation of a gene which, therefore, should be tightly regulated by its promoter. One such gene therapy approach is directed to cancer and utilizes so-called “oncolytic adenoviral vectors.” (See, for example, U.S. Pat. No. 5,998,205 (Hallenbeck et al.) Oncolytic adenoviral vectors are adenoviral vectors that are tumor-specific and replication competent after infection of the target cell in the organism. In this approach, a gene that is essential for the replication of an adenoviral vector is regulated by a tissue-specific promoter and thereby provides for tissue-specificity of the replication of the vector. Thus, in this approach, the adenoviral vectors will specifically replicate and lyse tumor cells if the gene that is essential for replication is exclusively under the control of a promoter that is tumor-specific, and is not induced by additional genetic elements that are not tissue-specific.
It is an object of the present invention to provide viral vectors, such as, for example, adenoviral vectors, that allow for the specific and tight regulation of a gene of interest within the viral vector. In the context of oncolytic adenoviral vectors, it is a further object of the present invention to provide for vectors with a high degree of tissue specificity.